Many different kinds of circuits utilize operational amplifiers to achieve desired functionality. For example, application circuits utilize operational amplifiers arranged in negative feedback configurations to realize a predetermined gain or other mathematical function over a selected frequency range. To prevent instability when used in a negative feedback configuration, operational amplifiers typically include some frequency compensation scheme, whereby the phase shift of the feedback signal is tailored to prevent constructive interference with the input that, otherwise, would induce unwanted oscillations.
One operational amplifier architecture and related compensation scheme that is used is a two-stage amplifier having a compensation capacitor connected between the output of the operational amplifier and the input of the second stage. This scheme is also generally known as Miller compensation. One problem with such an architecture, however, is that it may not provide enough loop gain at low frequencies for some applications, especially if implemented in reduced-scale semiconductor technologies, such as nanometer gate-length CMOS processes. Moreover, extending such an architecture to more than two stages, to increase the loop gain, typically results in a phase lag of 270° or more at the output, again presenting stability concerns.
Another operational amplifier architecture that can be used to provide higher loop gain is a multipath feedforward architecture. Such an architecture has a plurality of distinct amplification paths in parallel, typically ranging from low order amplification paths to higher order amplification paths. For example, one such architecture may include first, second, and third amplification paths arranged in parallel, the first-order path having a single amplifier, the second-order path having two amplifiers connected in series, and the third-order path having three amplifiers connected in series. Each amplification path typically contributes a different frequency response to the output, and some paths are designed to bypass or feedforward past other paths at selected frequencies. Feeding forward effectively removes the deleterious frequency impact of the bypassed stages at the selected frequencies, thereby enabling higher gain at these frequencies without as much concern over stability.
One advantage of multipath feedforward operational amplifiers is that they typically provide a higher loop gain in a selected frequency band without a corresponding high unity-gain frequency requirement that typically exists for a single-path architecture achieving the same gain in the selected frequency band. This characteristic often manifests as a steeper loop gain below the unity-gain frequency for multipath feedforward architectures in comparison to two- or multi-stage single-path architectures.
However, one problem with multipath feedforward operational amplifiers is that they may be inefficient from both chip-area and power-consumption perspectives. The large number of amplifiers required for many different independent amplification paths requires both a large chip area to implement and a large amount power to operate. Another problem with multipath feedforward architectures is that steeper loop gains below unity-gain frequency and corresponding phase shifts, which are typically associated with these architectures, may provide unintended or unavoidable frequency compensation effects due to the parallel nature of the feedforward amplification paths, which may again lead back to stability concerns, especially in view of the increased gains achieved by these amplifiers.
Thus, there exists a need for a multipath feedforward operational amplifier that is more efficient from size and power perspectives, but which can also achieve high loop gain and stability performance.